Gravity-driven Membrane Filtration Research Articles

Microplastics affect membrane biofouling and microbial communities during gravity-driven membrane filtration of primary wastewater

Recently, gravity-driven membrane (GDM) filtration has been adopted as an alternative solution for decentralized wastewater treatment due to easy installation and maintenance, reduced energy and operation cost, and low global warming impact. This study investigated the influence of microplastic size (0.5–0.8 μm and 40–48 μm) and amount (0.1 and 0.2 g/L) on the membrane performance and microbial community in GDM systems for primary municipal wastewater treatment. The results showed that dosing microplastics in the GDM systems led to 9–54% lower permeate flux than that in the control. This was attributed to more cake formation (up to 6.4-fold) with more deposition of extracellular polymeric substances (EPS, up to 1.5-fold) and divalent cations (up to 2.1-fold) in the presence of microplastics, especially with increasing microplastic amount or size. However, the dosed microplastics promoted formation of heterogeneous cake layers with more porous nature, possibly because microplastics created void space in the cake and also tended to bind with divalent cations to reduce EPS-divalent cations interactions. In the biofilm of the GDM systems, the presence of microplastics could lower the number of total species, but it greatly enhanced the abundance of certain dominant prokaryotes (Phenylobacterium haematophilum, Planctomycetota bacterium, and Flavobacteriales bacterium), eukaryotes (Stylonychia lemnae, Halteria grandinella, and Paramicrosporidium saccamoebae), and virus (phylum Nucleocytoviricota), as well as amino acid and lipid metabolic functions. Especially, the small-size microplastics at a higher dosed amount led to more variations of microbial community structure and microbial metabolic functions.

Biological pretreatment coupled gravity-driven membrane filtration for purifying decentralized domestic wastewater toward resource reuse

Membrane bioreactor (MBR) is an effective technology for the decentralized wastewater treatment; however, its high energy consumption and complex operation & maintenance limit its intensive application. This study introduced an innovative membrane-based process, gravity-driven membrane (GDM) filtration, to treat the decentralized wastewater without any cleaning procedures, and different biological pretreatments were constructed, including (I) tubular filler (TF-8), (II) tubular filler (TF-4), (III) fiber bundle filler (FBF), and (IV) sole aeration (control) to improve the filtration performance. The results indicated that the flux of GDM control can be maintained during long-term filtration in treating the real decentralized wastewater, and introducing the biological pretreatment processes could improve the stabilized flux of GDM without any influence on its stability. The average flux of TF-8/GDM was 3.69 ± 0.89 L m−2 h−1 during long-term filtration. Furthermore, coupling pre-treatment to GDM can effectively improve permeability quality and withstand the shock of water quality fluctuations. The average removal rates of chemical oxygen demand (COD) and dissolved organic carbon (DOC) by TF-8 were 59.7% and 48.3%, respectively. Besides, the organic contaminants could be further removed by the following GDM process owing to the biodegradation function of the self-formed biofilm on the membrane surface, and the DOC concentration in the GDM effluent can be reduced to as low as 4.5 mg L−1. Additionally, the elements of nitrogen (N) and phosphorus (P) could be effectively retention through the coupling process, with a retention rate of 84.3%–94.6% for total nitrogen (TN) and 99.8%–104.8% for total phosphorus (TP), respectively. Therefore, these findings can benefit to develop of new strategies to provide stable effluent quality, resource recovery, and energy consumption reduction in the membrane-based decentralized domestic wastewater treatment.

Laundry Waste Filtration Performance at Ultra Low Pressure

Laundry waste pollution can be handled through the development of filtration with the Gravity Driven Membrane (GDM) method which is designed to follow the criteria of the unloading module design, low polarization control, cost-effective, separation process using thrust in the form of a strong pressure difference. This type of research was an experiment. This study aimed to determine the performance of laundry waste filtration at ultra low pressure using clean water permeability and detergent solution as standardization. GDM filtration performance was tested by measuring permeability at a pressure of 1-10 kPa with two repetitions and long-term filtration for 35 days at a pressure of 6 kPa, the sample was replaced every 7 days without membrane washing. The results showed that as the pressure increases and the filtration time increases, the permeability of laundry waste decreases, this phenomenon is caused by compaction and clogging of the membrane pores (fouling). Permeability decreases from 1 kPa to 10 kPa pressure, the same applies to repetition. Long-term filtration permeability decreased significantly from the first day to the eighth day of filtration respectively 707.9 , 151.7, 115.8, 104.4, 97.5, 107.1, 102.1 and 103. 0 L/m2.hour.bar. On the 9th to 35th day the decrease was stable, this was caused by fouling on the membrane which resulted in the membrane becoming more compact and saturated. GDM filtration can take place for 35 days without washing the membrane with a permeability from the first day of 707.9 and the 35th day of 5.5 L/m2.hour.bar. The best performance of this filtration process lasts until the 9th day

Exploration of ferrate (VI) pretreatment and ultrasonication combined with gravity-driven membrane filtration for shale gas wastewater treatment

Gravity-driven membrane (GDM) filtration has gained attention as a promising technology for the decentralized treatment of wastewater. However, one major limitation of GDM filtration is its relatively low permeate flux. In this work, we investigated the application of GDM filtration, combined with ferrate (VI) pretreatment and ultrasonication, for the shale gas wastewater (SGW) treatment, and evaluated their respective impacts on the GDM performance. The results of our study revealed that ultrasonication played a significant role in enhancing the flux of GDM filtration, with an observed increase of 265%. This improvement can be attributed to the effective removal of pollutants from the membrane surface and the reduction of the fouling layer thickness caused by ultrasonication. Compared to a GDM alone, ferrate (VI) pretreatment could notably change the microbial community structure and reduce the extracellular polymeric substances accumulation, which may be linked with an enhancement in stable flux. Ferrate (VI) pretreatment selectively enriched particular microorganisms in SGW, which might perform a role in specific organics degradation. These findings provide valuable insights and suggestions for enhancing the overall performance of GDM systems in wastewater treatment. Notably, the ultrasonically enhanced GDM system, which confers the characteristics of simple operation, low energy supply and no chemical pollution, has great application potential in wastewater treatment.

Pre-ozonation for gravity-driven membrane filtration: Effects of ozone dosage and application timing on membrane flux and water quality

Gravity-driven membrane (GDM) filtration has been proposed as an alternative to sand filtration for drinking water treatment, but its low membrane flux and poor permeate quality hinder its widespread application. This study investigated the effect of pre-ozonation (with various dosages and application timings) on membrane flux, fouling layer characteristics, and permeate quality during GDM filtration of river water. Pre-ozonation with 3 mg/L ozone increased the mean membrane flux by 9–54%, resulting in approximately 30% increased water production and reduced dissolved organic carbon (DOC) and extracellular polymeric substance (EPS) production by 12–35% and 9–18%, respectively. The timing of pre-ozonation significantly affected the time-dependent flux behavior and morphological characteristics of the fouling layers. Pre-ozonation from the beginning of GDM filtration enhanced the initial flux because of organic fouling control but showed a gradual flux decline with time due to EPS accumulation and biologically inactive fouling layers. Pre-ozonation from the middle of GDM filtration period created heterogeneous and porous membrane fouling layers with high biological activity, showing increased membrane fluxes and improved permeate quality by removing DOC and assimilable organic carbon (AOC) via the combined action of ozone oxidation and biodegradation/sorption in the fouling layer. Overall, our results demonstrate pre-ozonation as a potential option to overcome the limitations of GDM filtration.

Gravity-driven membrane reactor for decentralized wastewater treatment: Comparison of reactor configuration and membrane module

Gravity-driven membrane (GDM) filtration is an attractive option for decentralized wastewater treatment due to lower energy consumption. However, insufficient information is available on the design factors of GDM reactor. In this study, we investigated the impacts of membrane aeration, types of membrane modules, and biocarrier deposition onto the membrane on water quality and membrane performance of GDM reactors. In detail, the membrane air diffuser located inside the granular activated carbon (GAC)-biofilm zone had benefit for mass transfer of dissolved oxygen and achieving co-existence of anoxic and aerobic conditions in the reactor, leading to higher removal efficiencies for organics (93.8–94.6 %), nitrogen (38.2–47.2 %) and phosphorus (31.9–57.7 %), compared to the air diffuser located outside the GAC zone (90.3 % for organics, 25.3 % for nitrogen and 7.0 % for phosphorus). In addition, the inside-out ceramic hollow fibre membrane showed ∼150–200 % higher fouling resistance (mainly cake resistance) due to greater biopolymer deposition compared to the outside-in polymeric hollow fibre membrane. However, for the outside-in flat sheet configuration, the ceramic and polymeric membranes showed comparable fouling resistance. The deposition of GAC particles on the membrane surface as secondary layer could improve permeate water quality, but at the expense of an increase in irremovable fouling.

Integrating gravity-driven ceramic membrane filtration with hydroponic system for nutrient recovery from primary municipal wastewater

In this study, a gravity-driven membrane (GDM) filtration system and hydroponic system (cultivating basil and lettuce) were combined for nutrient recovery from primary municipal wastewater. The GDM system was optimized by increasing the periodic air sparging flow rate from 1 to 2 L/min (∼15 hr per 3–4 days), resulting in a ∼52% reduction of irreversible fouling. However, the total fouling was not alleviated, and the water productivity remained comparable. The GDM-filtrated water was then delivered to hydroponic systems, and the effects of hydroponic operation conditions on plant growth and heavy metal uptake were evaluated, with fertilizer- and tap water-based hydroponic systems and soil cultivation system (with tap water) for comparison. It was found that (i) the hydroponic system under batch mode facilitated to promote vegetable growth with higher nutrient uptake rates compared to that under flow-through feed mode; (ii) a shift in nutrient levels in the hydroponic system could impact plant growth (such as plant height and leaf length), especially in the early stages. Nevertheless, the plants cultivated with the GDM-treated water had comparable growth profiles to those with commercial fertilizer or in soils. Furthermore, the targeted hazard quotient levels of all heavy metals for the plants in the hydroponic system with the treated water were greatly lower than those with the commercial fertilizer. Especially, compared to the lettuce, the basil had a lower heavy metal uptake capability and displayed a negligible impact on long-term human health risk, when the treated water was employed for the hydroponic system.

Using Stormwater in a Sponge City as a New Wing of Urban Water Supply—A Case Study

Rapid and even disruptive innovations are needed to make cities fit for the future. The particular challenge will be to transform existing urban spaces in order to increase climate resilience. Along these lines, rainwater harvesting has taken place insufficiently to date, even when Sponge City concepts are implemented. Thus, the concept presented here addresses existing urban neighborhoods and proposes to collect rainwater from nearby rooftops and treat it in decentral treatment units called “City Water Hubs” (CWH) equipped with modular coupled low-energy technologies to produce various customized “City Water” qualities, and store it until it can be used or distributed. A feasibility study with a focus on the campus area at the main building of the Leibniz University of Hannover, the determined rainwater qualities, and the results from investigations with two laboratory test plants provided the basis for the technical design of the pursued concept. The feasibility study showed how sufficient rainwater for irrigation purposes can be made available for the listed large university park even under extreme dry and heat wave conditions. If large portions of the roof area (11,737 m2) of the university’s main building were activated, even in a dry year with only 49.8% of the average precipitation, only 19.8% of the harvested stormwater would be needed for irrigation. The rainwater samples showed TSS concentrations of up to 7.54 mg/L, COD of up to 58.5 mg/L, and NH4 of up to 2.21 mg/L, which was in line with data reported in the literature. The treatment technologies used for the two pilot plants are proven approaches for stormwater treatment and were composed as follows: (1) gravity-driven membrane filtration (GDM) and (2) slow sand filter with integrated activated carbon (AC) layer. The treatment with both (1) and (2) clearly improved the rainwater quality. The GDM reduced turbidity by 90.4% and the Sand/AC filter by 20.4%. With regard to COD, the studies for GDM did not show a clear elimination trend; the Sand/AC filter reduced the COD by 77%. Taken together, decentralized low-energy rainwater treatment can reliably provide quality-assured City Water for any specific use. Regarding the treatment design, GDM is preferable and can be better operated with downstream UV disinfection, which might be needed to reduce the pathogenic load, e.g., for local heat control measures. The research steps presented here will pioneer the development of a city-wide rainwater harvesting infrastructure on the way of establishing stormwater as a resource for a new wing of urban water supply. The presented findings will now result in the implementation of a full-scale CHW on the campus to ensure long-term irrigation of the listed park, relieving the public drinking water supply.

Performance of gravity-driven membrane filtration for treatment of low-temperature and low-turbidity water: Effect of coagulation pretreatment

As a promising technology for decentralized drinking water treatment, performance of gravity-driven membrane (GDM) filtration for low-temperature and low-turbidity water purification and effects of coagulation pretreatment were investigated in this study. Flux development and permeate water quality of three GDM systems, i.e., control GDM without pretreatment, GDM with in-line coagulation (IC) pretreatment (IC-GDM), GDM with coagulation-sedimentation (CS) pretreatment (CS-GDM), were compared. Moreover, composition and morphology of bio-cake layer were characterized to explain underlying mechanisms for performance improvement by coagulation pretreatment. Results suggested that permeate flux of control GDM stabilized at ∼4.0 L/(m2·h) for raw water with relatively low temperature (8.1– 10.8 °C) and turbidity (1.15– 2.47 NTU). IC pretreatment increased stable flux by 65 %, although the bio-cake layer thickness in IC-GDM was increased by 71.7 %, probably due to a significant reduction (57.1 %) in extracellular polymeric substances (EPS) in the bio-cake layer. Meanwhile, CS pretreatment increased stable flux by 87.5 %, which can be attributed to 63.3 % reduction in EPS and 47.0 % decrease in bio-cake layer thickness. As for permeate water quality, dissolved organic matter (DOM) was significantly reduced by both IC and CS pretreatments, mainly due to transformation from dissolved state to particulates. Compared with control GDM, both IC and CS pretreatments resulted in decrease in microbial abundance and activity and changes in microbial community, probably due to decrease in bioavailable phosphate and DOM. This study suggested that both IC and CS pretreatments can be integrated with GDM filtration for efficient purification of low-temperature and low-turbidity water.

Influence of pressure in a point-of-use treatment of well water using gravity-driven ultrafiltration membranes for drinking purposes in developing countries

The use of polymeric membranes that do not require electricity and cleaning is an attractive technology for use in the point-of-use (POU) treatment of water in areas where access to drinking water is limited. This study sought to evaluate the performance of a biofilm-controlled gravity-driven membrane (GDM) in the treatment of well water contaminated with fecal microorganisms and to validate the operating and maintenance concepts described in the literature. The effects of different pressures (50, 70, and 100 mbar) were tested using a 100,000 Da ultrafiltration (UF) membrane across 129 days of continuous filtration. After stabilization, the system achieved average permeate fluxes of 4.4 ± 0.2 L m−2 h−1, 4.7 ± 0.3 L m−2 h−1, and 6.5 ± 0.4 L m−2 h−1, and total membrane resistances of 4.2 × 1012 m−1, 5.4 × 1012 m−1, and 5.5 × 1012 m−1 for the hydraulic loads of 50, 70, and 100 mbar, respectively. An analysis of the filtrates showed a 3-log removal efficiencies for E. coli in all filters, producing water that was in compliance with potability standards. The average turbidity removal efficiencies were 72 %, 74 %, and 50 % for hydraulic loads of 50, 70, and 100 mbar, respectively. The total organic carbon content and the ultraviolet absorption at 254 nm were reduced by 7–17 % and 10–30 %, respectively. The membrane did not affect the removal of chlorides and sulfates. The biofilm, developed in an oligotrophic environment, contributed to the filtration of a constant amount of water (flux stabilization) but did not improve the quality of the filtrate. The results of this study suggest that the GDM filtration system can be considered as a potential alternative to produce POU decentralized drinking water in terms of both quantity and quality. The constant permeate flux and the complete removal of E. coli without the need for backwashing or cleaning make the technology attractive for application in developing countries to treat well water at low cost and minimum maintenance.

The improvement of heavy metals removal by wood membrane in drinking water treatment: Comparison with polymer membrane and associated mechanism

The application of commercial membranes is limited by the secondary pollution such as the usage of toxic chemicals for the membrane preparation and the disposal of aged membranes. Therefore, the green and environmentally friendly membranes are extremely promising for the sustainable development of membrane filtration in water treatment. In this study, the comparison of wood membrane with the pore size of tens microns (μm) and polymer membrane with the pore size of 0.45 μm was made to study the heavy metals removal in drinking water treatment by gravity-driven membrane (GDM) filtration system, and there was an improvement in the removal of Fe, Cu and Mn by wood membrane. The sponge-like structure of fouling layer for wood membrane made the retention time of heavy metals prolonged in contrast to the cobweb-like structure of polymer membrane. The carboxylic group (-COOH) content of fouling layer for wood membrane was greater than that for polymer membrane. Additionally, the population abundance of heavy metal-capturing microbes on the surface of wood membrane was higher compared with polymer membrane. The wood membrane provides a promising route to producing facile, biodegradable and sustainable membrane as a green alternative to polymer membranes in heavy metal removal from drinking water.

Influence of powdered activated carbon on gravity-driven ultrafiltration for decentralized drinking water treatment: Insights from microbial community and biofilm structure

Gravity-driven membrane (GDM) filtration, an appealing approach for decentralized drinking water treatment, tolerates the development of membrane biofilms for the benefit of low energy consumption and stable flux. In this study, the effect of introducing powdered activated carbon (PAC) on the biofilm generated during long-term GDM filtration was investigated. The results indicated that the addition of PAC significantly enhanced the removal of dissolved organic compounds (DOC) (31 %), whereas only 15.7 % removal was achieved in the control GDM system without PAC. The improved permeate quality was attributed to the combined effects of adsorption and biodegradation of organic compounds by PAC. However, the presence of PAC reduced the permeability of the GDM system by 39 % compared to the control one, which was related to the synergetic effect of the increased extracellular polymeric substances (EPS) content and the decreased metazoan amounts (especially Nematoda) in the biofilm. A large number of substrates adsorbed by PAC facilitated the growth of bacteria and hence intensified the secretion of EPS. Meanwhile, the deposition of PAC on the membrane surface limited the movement space of metazoans, leading to a denser and flatter biofilm basal structure. Furthermore, fewer metazoans meant less predation on bacteria, which also contributed to bacterial reproduction. In addition, an enhanced removal efficiency of ammonia nitrogen was observed in the PAC/GDM system, corresponding to a much higher abundance of Nitrospira in bacterial community. This study links microbial community to hydraulic resistance of biofilm and has practical implications for the application of GDM processes.

Ultrasonication-assisted fouling control during ceramic membrane filtration of primary wastewater under gravity-driven and constant flux conditions

This study investigated the feasibility of using ultrasonication as a ceramic microfiltration membrane fouling control strategy during gravity-driven membrane (GDM) filtration and constant flux filtration of primary municipal wastewater. The fouling model fitting curves revealed that initial intermediate pore blocking was dominant in the GDM system, followed by cake filtration; while cake filtration was dominant during constant flux filtration with a continuously increasing pattern. In the GDM system, periodic ultrasonication cleaning achieved higher cake resistance reduction (∼60 % vs ∼ 26 %) and led to lower specific cake resistances (0.2–2.0 × 107vs 29.6–59.3 × 107 m/mg) compared to periodic backwash. Ultrasonication-facilitated fouling mitigation effectiveness was related to cake properties (such as cake foulant density and porosity) and improved with extending ultrasonication duration. The foulant examination indicated that ultrasonication may result in porous cake nature by cake expansion and facilitate detaching particulate foulants and soluble organics (such as biopolymers) from the membrane. Cost analysis revealed that with commercial ceramic membranes, ultrasonication-facilitated GDM systems are economically feasible for treating wastewater at a household scale (∼1.1 EUR/m3 water), while ultrasonication-facilitated constant flux filtration systems are favorable at a small-community scale (∼0.5 EUR/m3 water). Furthermore, it was predicted that the use of low-cost recycled ceramic membranes would allow GDM systems as economical decentralized wastewater treatment processes due to their lower water production cost (∼0.16 EUR/m3 water) with easy operation and maintenance.

Gravity-driven membrane filtration with compact second-life modules daily backwashed: An alternative to conventional ultrafiltration for centralized facilities

Gravity-driven membrane (GDM) filtration is a strategic alternative to conventional ultrafiltration (UF) for the resilient production of drinking water via ultrafiltration when resources become scarce, given the low dependency on energy and chemicals, and longer membrane lifetime. Implementation at large scale requires the use of compact and low-cost membrane modules with high biopolymer removal capacity. We therefore evaluated (1) to what extent stable flux can be obtained with compact membrane modules, i.e., inside-out hollow fiber membranes, and frequent gravity-driven backwash, (2) whether we can reduce membrane expenses by effectively utilizing second-life UF modules, i.e., modules that have been discarded by treatment plant operators because they are no longer under warranty, (3) if biopolymer removal could be maintained when applying a frequent backwash and with second-life modules and (4) which GDM filtration scenarios are economically viable compared to conventional UF, when considering the influence of new or second-life modules, membrane lifetime, stable flux value and energy pricing. Our findings showed that it was possible to maintain stable fluxes around 10 L/m2/h with both new and second-life modules for 142 days, but a daily gravity-driven backwash was necessary and sufficient to compensate the continuous flux drop observed with compact modules. In addition, the backwash did not affect the biopolymer removal. Costs calculations revealed two significant findings: (1) using second-life modules made GDM filtration membrane investment less expensive than conventional UF, despite the higher module requirements for GDM filtration and (2) overall costs of GDM filtration with a gravity-driven backwash were unaffected by energy prices rise, while conventional UF costs rose significantly. The later increased the number of economically viable GDM filtration scenarios, including scenarios with new modules. In summary, we proposed an approach that could make GDM filtration in centralized facilities feasible and increase the range of UF operating conditions to better adapt to increasing environmental and societal constraints.

Impact of Gravity-Driven Membrane Filtration Water Treatment Systems on a Rural School in Indonesia

Improving access to safe drinking water in developing countries is still a challenge and Gravity-Driven Membrane (GDM) filtration systems may be a sustainable solution. Two rural schools in West Java Indonesia were studied, one as a control site and another having an installed GDM system. Chemical and microbiological water quality data were collected for an initial 3-month period at the GDM site and a final sampling at the study’s conclusion (6 months) at both sites. After the initial 3-month period, health surveys were conducted with students self-reporting incidences of diarrhea for 3 months at both school sites. An analysis of the chemical parameters indicated that both schools had good water quality. An average 2-log reduction of fecal indicator bacteria at the GDM site was observed, with the control site having numbers that exceeded the upper detection limits (>3.38 log CFU/100 mL). Student diarrhea incidence at the GDM site declined from 0.077 at the survey onset to 0.052 at the latter half of the survey period, while the control site had a diarrhea incidence of 0.077 throughout. The results indicate that GDM technology can serve as a practical water filtration technology, improving access to safe drinking water for rural populations.

Effects of organics concentration on the gravity-driven membrane (GDM) filtration in treating iron- and manganese-containing surface water

Iron and manganese contamination in the surface water is posing great challenges to the drinking water treatment supply, especially in the complex cases of organics involvement. Gravity-driven membrane (GDM) filtration equipped with the dual functions of ultrafiltration and biocake layer, conferred promising potentials in the removals of iron and manganese. This study evaluated the effects of organics concentrations on the removal performance of iron and manganese, as well as on the flux stabilization during GDM long-term filtration. The results indicated that stable flux level and the removal efficiency of manganese initially increased with the increase of organics concentration in the feed water, and then decreased. The moderate concentration of organic compounds in the feed water would positively facilitate the microbial activities and benefit to engineering a heterogeneous and porous biocake layer on the membrane surface, contributing to the highest improvements of stable flux (6.3 L m−2 h−1), while high concentration of organic compounds in the feed water would result in the increase in the thickness and EPS concentration of the biocake layer, leading to a flux reduction. Furthermore, the moderate concentration of organic compounds in the feed water was also beneficial to the manganese removal (> 94.6%) due to the more accumulation of auto-catalytic oxidation manganese oxides (MnOx) within the biocake layer and the improved biological degradation, however, further increase of organics concentration would deliver a negative impact on the manganese removal owing to the wrapping of MnOx by the organic substances. Overall, these findings provide practical and acceptable strategies to the selections of pre-treatments prior to GDM and promote its extensive application in treating the iron- and manganese-containing surface water.

Effects of pre-treatments on the filtration performance of ultra-low pressure gravity-driven membrane in treating the secondary effluent: Flux stabilization and removal improvement

Gravity-driven membrane (GDM) filtration is a promising technology for decentralized water and wastewater treatment due to its negligible energy consumption and low maintenance. This study investigated the flux development and permeate quality of the GDM process in treating the secondary effluent, as well as the influences of two practical pre-treatments, including GAC filter and microfiltration (MF), on its filtration performance. The results indicated that the GAC filter could significantly enhance the removal of organic contaminants (i.e., DOC, UV254, and fluorescent substances), with an average removal of 70 % for DOC and 80 % for UV254, respectively. Meanwhile, GAC/GDM systems exhibited a superior removal performance of manganese and sulfamethoxazole (SMX) for more than 95 % and 80 %, respectively. Furthermore, the optimized mixing ratio between GAC permeates and secondary effluent was 7:3 (GAC7:3) to reach a flux level close to the GDM control and achieve a superior permeate water quality than the other processes. The flux of GDM control could stabilize at the level of 5.47 ± 0.71 L m−2h−1 during long-term filtration. Unexpectedly, limited flux improvements were observed with the pre-treatments, and the stable flux of GAC/GDM and MF/GDM was close to or slightly lower than the GDM control. In GDM control, a rough, heterogeneous, and porous biocake layer was observed on the membrane surface. By contrast, the cake layer structures in the GDM system coupled with pre-treatments appeared denser, compact, and homogeneous, resulting in relatively lower stable flux relative to the GDM control. The economic assessment indicated that the optimized process of GAC7:3 showed a significant cost advantage in the small and medium-scale water reuse scenarios. These findings were beneficial to develop new energy-saving membrane-based technology for the reclamation of wastewater.

Insight into the role of biogenic manganese oxides-assisted gravity-driven membrane filtration systems toward emerging contaminants removal

Effective water purification technologies are required to remove emerging contaminants (ECs) and prevent their extensive occurrence in rural areas. In this work, coupling gravity-driven membrane (GDM) filtration with biogenic manganese oxides (BioMnOx) in the biofouling layer was utilized for treating water containing SMX. Comparisons between BioMnOx-GDM (with BioMnOx) and Control-GDM (without BioMnOx) indicated that BioMnOx could significantly promote the removal of DOC, NH4+-N, and fluorescent pollutants due to its strong oxidating capacity and high biological activity. The formation of BioMnOx increased the abundance of SMX-degrading bacteria, enriched the metabolic pathway and mineralization rate of SMX, and effectively promoted the remove of SMX. More importantly, BioMnOx facilitated the removal of antibiotic resistance genes (ARGs) in the GDM, because it increased the link between microorganisms and reduced the concentration of SMX, thus reduced the expression of ARGs. LB-EPS played an important role in the membrane fouling. Compared with the Control-GDM, the concentration of LB-EPS in BioMnOx-GDM decreased, which was beneficial to alleviate membrane fouling. Although a thicker biofouling layer (1774.88 μm vs.775.54 μm) was formed in BioMnOx-GDM, the biofilm with higher porosity (64.93% vs. 41.24%) had a more positive effect on the flux. Overall, BioMnOx could improve the pollutant removal and stable flux level of the GDM system. BioMnOx-GDM effectively avoided the risks brought by ECs and ensured water safety in rural areas.

Compaction of a Polymeric Membrane in Ultra-Low-Pressure Water Filtration.

Applications of ultra-low-pressure filtration systems are increasing as they offer enhanced sustainability due to lower energy input, almost no use of chemicals, and minimum operational expenditure. In many cases, they operate as a decentralized system using a gravity-driven membrane (GDM) filtration process. These applications are relatively new; hence, the fundamental knowledge of the process is still limited. In this study, we investigated the phenomenon of polymeric membrane compaction under an ultra-low-pressure system. The compaction phenomenon is well-recognized in the traditional pressure-driven system operating at high transmembrane pressures (ΔPs > 200 kPa), but it is less documented in ultra-low-pressure systems (ΔP < 10 kPa). A simple GDM filtration setup operated under a constant-pressure system was employed to investigate the compaction phenomena in a polymeric hollow fiber membrane for clean water filtration. Firstly, a short-term pressure stepping test was performed to investigate the occurrence of instantaneous compaction in the ΔP range of 1–10 kPa. The slow compaction was later investigated. Finally, the compaction dynamic was assessed under alternating high and low ΔP and relaxation in between the filtrations. The findings demonstrated the prominence of membrane compaction, as shown by the decreasing trend in clean water permeability at higher ΔPs (i.e., 3240 and 2401 L m−2 h−1 bar−1 at ΔPs of 1 and 10 kPa, respectively). We also found that the intrinsic permeability of the applied polymeric membrane was significantly higher than the apparent one (4351 vs. 2401 L m−2 h−1 bar−1), demonstrating >50% loss due to compaction. The compaction was mainly instantaneous, which occurred when the ΔP was changed, whereas only minor changes in permeability occurred over time when operating at a constant ΔP. The compaction was highly reversible and could be restored (i.e., decompaction) through relaxation by temporarily stopping the filtration. A small fraction of irreversible compaction could be detected by operating alternating filtrations under ΔPs of 1 and 10 kPa. The overall findings are essential to support emerging GDM filtration applications, in which membrane compaction has been ignored and confounded with membrane fouling. The role of compaction is more prominent for high-flux GDM filtration systems treating less-fouling-prone feed (i.e., rainwater, river water) and involving membrane cleaning (i.e., relaxation) in which both reversible and irreversible compaction occurred simultaneously.

Removal of polystyrene nanoplastic beads using gravity-driven membrane filtration: Mechanisms and effects of water matrices

Nanoplastics (NPs) are emerging water contaminants and the smaller particle size of NPs further hinders the remediation in the aspects of energy consumption and removal efficiency. Due to the loose structure and tunable functionalities, nanofibrous membranes, with ultrathin fibers (73.3 ± 31.9 nm), were developed for NPs removal in a gravity-driven application. With an ultralow ΔP0 of 353 Pa, greater than92 % of model polystyrene NPs (average size from 107 to 1450 nm) were removed at an average water flux of 109 Lm-2h−1. With advanced material characterization, the removal mechanisms were determined as: (1) size-exclusive membrane interception, which dominates the removal when dNPs > dpores; and (2) membrane adsorption of smaller NPs via electrostatic and hydrophobic interactions. The distinct removal mechanisms trigger different long-term performances on removal and water flux. Complex water matrices were also evaluated, where acidic condition and anionic surfactant were observed to hinder the removal. In addition, the nanofibrous membranes showed resistance towards inorganic scaling whereas organic foulants decreased the water flux by 15.6 % after 40 consecutive filtration runs. The fouled membranes can be regenerated in mild conditions with 5 % ethanol. Overall, nanofibrous membranes are effective in NPs removal with the high-removal efficiency, low-fouling tendency, and ease of regeneration.